Useful Educational Guides
Not too many years ago the only status indicators we had in our automobiles were a coolant temperature gauge, a voltage or battery charge indicator, a speedometer and an oil pressure gauge if we were lucky. Today we can also monitor our engine speed and diagnostics, brake pad condition, tyre pressures, fuel consumption, air bag condition, seat belt usage, windscreen washer bottle fluid level as well as progress towards our destination, and probably several more parameters besides. Some components of our vehicle however remain difficult or impractical to monitor such things as the state of the camshaft drive belt or the brake fluid condition for example. In these cases, the component is usually ‘lifed’ i.e. it is recommended for renewal or overhaul after a set period of time or mileage. In order to comply with this recommended maintenance however, and also to schedule routine servicing perhaps, we obviously need to monitor the vehicle’s use with a mileometer which provides us with a readout of the total distance travelled from new or from the last service.
A similar situation exists with hydraulic systems; not too many years ago the only instrumentation fitted to a hydraulic power unit would probably be a fluid thermometer, a sight glass and a pressure gauge. If we were lucky and the system was relatively new, the pressure gauge may even have worked, but more often than not its needle would be found lying at the bottom of the dial, the victim of pressure spikes or mechanical vibration.
Nowadays however, we are able to reliably monitor not only system pressure and pressure transients, but also fluid flow rate, temperature, cleanliness and water content, component vibration, efficiency etc. etc. As industrial and mobile machinery has become increasingly sophisticated, such predictive maintenance components have become both more readily available and cost effective as well as being more necessary. An excavator standing idle due to a burst hose or an unforeseen pump failure on an injection moulding machine could be costing thousands of pounds per hour in lost production alone. Not only are unexpected component failures likely to be costly, they may also be potentially dangerous and possibly environmentally polluting as well.
But just as with our automobiles, there remain a few hydraulic components whose condition is difficult to monitor effectively. Flexible hose condition is one example. Although at least one manufacturer now has the ability to monitor hose condition, it is still far from being available for all hose applications. As a result, hydraulic hoses together with the elements of simpler filters and air breathers, dynamic seals on cylinders etc. are recommended to be changed after specified periods of either time or usage. Although the passage of time is easily established, machine usage is not always so easily monitored. For a process machine operating continuously two or three shifts a day then usage can be predicted reasonably accurately. The same is not the case however with a piece of farm or construction machinery which is only used intermittently or seasonally. In other words, in most cases, hydraulic systems still lack an equivalent of a vehicle’s mileometer. That situation can now be easily remedied however with the new RFS200 run-time monitor product from Webtec.
Developed from their extensive range of flow measuring devices, the principle of operation of the Webtec unit is by means of a variable orifice flow meter and magnetic switch which senses the position of a piston. Sensing flow rather than pressure means that trapped-in pressure or pressure created by reactive loads (or thermal expansion) when the system is shut down cannot generate a false usage reading. The unit can be installed in any part of the hydraulic system and is capable of operating with pressures up to 420 bar. Flow rates up to 200 L/min can be passed through the unit with minimal flow resistance; for example, with a forward flow rate of 200 L/min the pressure drop through the unit would typically be less than 3 bar. It is also possible to reverse flow through the unit, although counting does not occur in the reverse flow direction.
The unit is suitable for use with hydraulic mineral oil to ISO 11158 category HM, with an operating fluid temperature range from -20°C to 100°C and timer accuracy is ±0.2% over the specified temperature range. Different port connection sizes and thread forms are available to match with customer requirements. The unit should be mounted horizontally and not installed immediately adjacent to components generating high magnetic fields (such as an electric motor). It should be protected by at least a 40 micron filter in the hydraulic circuit and oil cleanliness should be maintained better than NAS 8 or ISO 19/17/14. There are two M6 holes for bulk-head mounting but these should not be used to support pipework. Flexible hoses connected to the unit should be clamped to minimise flexing stress at the threaded ports. Needless to say, all connections should be made by suitably trained personnel.
The unit is pre-set so that counting is initiated when flow through the unit exceeds 10 L/min. This trigger point is unaffected by pressure but may vary slightly with fluid viscosity - higher viscosities will decrease the switching flow and lower viscosities will increase the switching flow. Once the trigger point is reached, a blinking decimal point on the LCD display indicates that the counter is operational and counting accumulates while the flow rate remains above the trigger point. Being battery operated (with sensitive components protected to IP66), the unit requires no external power supply and battery life is stated to be at least 10 years. No reset function is provided on the unit so that it is effectively tamper-proof, although the unit could obviously be replaced if someone was sufficiently determined to falsify any run-time data. Currently the data output is a visual readout but other options (such as an IoT cellular connection) for remote data acquisition could be included depending upon demand.
Initially the product was market driven following a request from a customer manufacturing hydraulic attachments in the agricultural sector. Many such attachments are shared by co-operatives so a foolproof way of charging customers by usage was required. Another customer in the construction machine business needed to monitor usage for each attachment in order to apply an appropriate maintenance schedule after 50, 100 or 500 hours of use. However, each attachment may be changed 30 times a day and keeping track of their usage any other way would be very difficult.
In the industrial sector, applications requiring high flow rates of fluid, often use multiple pump systems where individual pumps can be brought on line at the appropriate time in the machine’s flow cycle. In some cases, additional pumps may be reserved purely as stand-by units which may only be used in the event of a failure of one of the normal duty pumps. Such arrangements are common in process industries (such as steelworks applications for example), where a malfunction of the hydraulic system would cause major problems. Being able to keep track of each individual pump’s usage would therefore be a significant benefit in planning a scheduled maintenance programme for pump overhaul or replacement. The information could also be used to even out the run time of all the pumps used on a system if necessary. One school of thought suggests that this is not always the correct approach however since all pumps then approach the end of their useful life at approximately the same time. But by having the relevant usage data for each pump, the maintenance engineer can at least make the most appropriate decision with regards to how the pumps are utilised.
For fixed displacement pump systems using unloading valves, the unit could be installed on the system side of the unloading valve to record only the time that the pump is delivering flow at full pressure rather than the unloaded periods when minimal wear is likely to take place. Alternatively, the unit could be installed on the pump side of the unloading valve in which case it would record the total pump run time both on and off load. Once again, the choice can be determined by the preferences of the maintenance engineer or end customer.
The condition of hydraulic motors is not always easy to establish using on-board diagnostic sensors, so in applications where they operate for lengthy periods of time, (such as mixer, shredder or conveyor drives), a usage-based replacement or overhaul schedule can be established using data provided by a run-time monitor. Component test rigs may also be an application where the unit may prove useful. Endurance testing often involves operating or cycling components for hundreds or thousands of hours so the unit’s capabilities to accurately record up to almost 10 million hours of operation will cover most requirements.
So for little more than the cost of a tankful of diesel, the Webtec RFS200 unit can provide a useful device for:
For more information on the RFS200 Run-Time Monitor, please visit www.webtec.com
This article was originally published in the August 2021 issue of Fluid Power Journal and is reposted with permission